单项选择题

______ caused the accident is still a complete mystery.

A．How
B．That
C．Where
D．What
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单项选择题
By this time tomorrow we ______ the machine.
A．have repaired
B．will repair
C．shall have repaired
D．would repair
单项选择题
I am sorry ______ you so much trouble. And thank you for your help.
A．to give
B．giving
C．to have given
D．gave
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